This invention relates generally to techniques for measurement of bone mineral content, and, more particularly, relates to non-invasive methods and apparatus for accurate and consistent measurement of bone mineral content and density.
Methods for non-invasive measurement of bone mineral content (BMC) and density provide an important diagnostic technique in evaluating the state of an individual's bone structure, and particularly, in determining whether an individual suffers from osteoporosis, or is undergoing bone loss which can eventually lead to osteoporosis if not detected and controlled. Measurement of BMC values is especially important in older individuals, for evaluating the efficacy of treatment for osteoporosis.
One widely employed technique for measuring BMC at selected regions of bone is bone absorption densitometry, which involves measurement of the attenuation of radiation--such as X-rays-- transmitted or scanned through a selected bone region in an individual. The magnitude of attenuation of the radiation is directly related to the quantity of bone mineral in the selected bone area, and attenuation values can be generated directly from radiological image data.
Bone tissue continually undergoes a remodeling process in which old bone dissolves and new bone forms in its place. The rate of this remodeling process depends upon the composition of the bone. Human bones, for example, are composed of two bone types, referred to as cortical bone and trabecular bone. Cortical bone has a relatively high density, and is found principally on the outer surface of bone structures. Trabecular or cancellous bone is porous, has a lower density than does cortical bone, and is found primarily in the core regions of bones. The amount of trabecular and cortical bone in the radius and ulna of the forearm, for example, has been well documented. See, for example, R. A. Schlenker et al, Calcif. Tiss. Res., Vol. 20, pp. 41-52 (1976).
Because trabecular bone has a remodeling or turnover rate 7 to 8 times greater than that of cortical bone, the sensitivity of bone-loss detection increases in proportion to the percentage of trabecular bone in the measurement region. Sites with higher percentages of trabecular bone can therefore provide more accurate indications of the magnitude of bone loss. Trabecular BMC values are typically measured at several bone measurement sites to determine whether bone loss is occurring. These sites commonly include the lower vertebrae of the spine, including lumbar 1-3, the hip, the heel, and the distal area of the radius and ulna--i.e., the area closest to the wrist. Each of these bone sites contain varying amounts of trabecular and cortical bone.
In particular, the distribution of cortical and trabecular bone throughout the human skeletal system is inhomogeneous. The heel bone, Os Calcis, is 95% trabecular in content, while the intermediate regions of the radius and ulna contain 95%-96% cortical bone. The percentages of cortical and trabecular bone, moreover, can vary greatly in the same bone from one area to another. This variation limits the accuracy of conventional bone absorption densitometry, because cortical and trabecular bone have significantly different densities and thus different X-ray attenuation. Using Iodine-125 as an X-ray source, for example, the difference in transmitted 27 KeV photons through 1 mm of cortical bone and 1 mm of trabecular bone is approximately 28 percent. The varyinq amounts of cortical and trabecular bone in different regions of the same bone therefore cause spurious changes in calculated bone loss values when subsequent measurements are performed on different bone measurement regions.
While bone absorption densitometry has proven useful for generating estimates of BMC values, conventional bone absorption densitometry methods and apparatus are hampered by several sources of error which limit accuracy and resolution. These errors-- which arise from variability in bone type from region to region, and the criteria utilized in selecting and defining bone measurement regions--are most apparent when practitioners attempt to evaluate bone loss over periods of months or years by correlating BMC values generated in different measurements or "scans".
Those skilled in bone absorption densitometry techniques have long recognized that precise definition of the selected bone measurement area is a prerequisite for accurate measurement of BMC. Practitioners have emphasized the need for defining and measuring the same bone area, without measurement region positioning error, in subsequent BMC scans. See, for example, Ross et al, J. Bone & Mineral Res., Vol. 3, pp. 1-11, (1988); L. Nials et al, J. Nuclear Med., Vol. 26, pp. 1257-1262 (1985); and B.J. Awbrey et al, J. Orthopedic Res., Vol. 2, pp. 314-321 (1984). Conventional bone absorption densitometry techniques, however, have not provided a means for eliminating measurement region positioning errors.
It is therefore an object of the invention to provide bone absorption densitometry methods and apparatus which provide precise and consistent selection of BMC measurement regions.
The deficiencies of conventional bone absorption densitometry methods and apparatus are especially apparent in diagnosing the gradual bone loss which is characteristic of osteoporosis. Bone loss in women, for example, is approximately 0.5% per year prior to menopause. Following menopause, women can lose bone at approximately 1%-2% per year and, in some cases, at much higher rates. Therefore, in order to determine whether an individual has a bone loss problem, BMC measurement techniques must be capable of detecting bone loss as small as 1%. Conventional techniques for measuring BMC values generally fail to achieve this level of precision required for accurate diagnosis of osteoporosis, due to an inability to establish a consistent measurement region of bone which can be equivalently evaluated in scans performed months or years after the initial scan.
It is thus another object of the invention to provide such methods and apparatus which can accurately and consistently select the same measurement region during successive scans.
Certain conventional techniques for measurement of BMC values utilize a predetermined value of the interosseous distance between the radius and ulna to define a bone measurement area. This interosseous distance criterion is another source of error, because inaccurate measurement of the interosseous distance results in significant BMC measurement variance. An error of approximately 3% in measured BMC values can arise if an initial BMC measurement of the radius bone of the forearm is performed at a region where the radius and ulna are separated by 5 mm, and a subsequent BMC measurement is made at a region having an interosseous separation of 4 or 6 mm. See B. J. Awbrey et al, J. Orthopaedic Res., VoI. 2, pp. 314-321 (1984). This error is due to the different cortical or trabecular bone percentages in the first and second measurement regions.
This interosseous distance, moreover, is not a constant, but instead is a variable which depends upon other parameters. For example, rotation of the wrist causes translation of the radius and ulna, resulting in variation of the interosseous distance. Rotation of the wrist, whether clockwise or counterclockwise, can occur even when the subject's wrist is clamped. This interosseous variability has been identified by a number of investigators in the field. See, for example, J.B. Christensen et al, Anta Rec ., Vol. 160, pp. 261-272 (1968).
In addition, the relative angular position of the forearm and upper arm, and the angular position of the humerus relative to the body, can have a significant effect on the interosseous distance. In view of the many possible variations and degrees of freedom in positioning a patient's forearm, and the varying level of expertise of the personnel executing a bone scan, it is highly unlikely that the same measurement region can be selected in a given instance on the basis of interosseous distance. It is still more unlikely that the same measurement region can be selected on this basis during successive scans over the course of time.
Those skilled in the art have long recognized this repositioning error, and the consequent inability to consistently select the same bone measurement region in successive scans.
It is therefore a further object of the invention to provide such methods and apparatus which can automatically correct repositioning errors and provide accurate correlation with previously measured BMC and other values.
Conventional BMC measurement methods provide no data representative of the exact position of the areas scanned by the x-ray source, relative to the imaged bone. Instead, the scanned area is correlated with the edges of each bone by utilizing an "edge criterion", under which a bone edge is "detected" when radiation attenuation has decreased by a selected percentage from the tissue baseline attenuation. This "edge criterion" limits sensitivity in identifying bone edges that are in close proximity. Moreover, such "edge criteria" are arbitrarily defined, and do not define measurement regions having high trabecular bone content--i.e., at least 60%--in the distal radius or ulna.
It is therefore a further object of the invention to provide bone absorption densitometry methods and apparatus which facilitate selection of measurement regions having high trabecular bone content.
It is still another object of the invention to provide such methods and apparatus which can be substantially automated, and which can be rapidly and easily utilized by an operator.